The basic operating pacing mode of a DDD/AAI pacemaker is an AAI pacing mode, with a single chamber atrial stimulation, together with and a contemporaneous monitoring (detection) of the ventricular activity. This AAI pacing mode is maintained as long as atrio-ventricular conduction is normal, i.e., as long as each atrial event (either an atrial detection or P-wave, corresponding to a spontaneous activity, or an atrial stimulation, or A-wave) is followed by an associated ventricular detection (a spontaneous activity or an R-wave).
In certain circumstances, however, atrio-ventricular blocks (AVB) can appear that are known as “paroxystic”, i.e., involving a temporary defect of the conduction that should lead to a spontaneous depolarization of the ventricle. In this case, the pacemaker commutes automatically to the DDD pacing mode, with cardiac rhythm management parameters that are optimized for this situation of temporary AVB. After disappearance of the AVB, and there is a re-establishment of spontaneous atrio-ventricular conduction, and, when a certain number of conditions are filled, the pacemaker turns again automatically to the AAI pacing mode (with ventricular surveillance).
A known DDD-AMC system for the automatic commutation of between DDD and AAI pacing modes is described, for example, in EP-A-0 488 904 and its counterpart U.S. Pat. No. 5,318,594) (commonly assigned herewith to ELA Médical), the disclosure of which U.S. Pat. No. 5,318,594 is incorporated by reference herein in its entity.
These automatic mode switching devices can be in particular implanted among patients that are suffering from sinusal dysfunctions that are likely to produce disorders or troubles of the atrial rate. The term “trouble of the atrial rate” or “TdAR” is a generic term that cover various atrial arrhythmias (non physiological episodes of acceleration of the rate) such as tachycardia, fibrillation, flutter, etc; troubles that are all characterized by the detection of a fast atrial rate. Primarily, one will consider that there is TdAR when the detected atrial rate exceeds an acceptable threshold level, this threshold level being eventually related to a degree of effort determined by a physiological sensor.
To reduce the episodes of atrial arrhythmia, the pacemaker can be equipped with a mode called “overdrive”, which is a particular mode ensuring an atrial stimulation at a frequency that is slightly higher than the subjacent natural rate. This mode of overdrive is described, for example, in European publication EP-A-0 880 979 and its corresponding U.S. Pat. No. 6,078,836 (commonly assigned herewith to ELA Médical).
The starting point of the present invention lies in a certain number of observations carried out at the time of a clinical follow-up of patients implanted with devices able to implement the two above mentioned functionalities, namely DDD-AMC and overdrive.
Indeed, the DDD-AMC mode of stimulation is reserved to the patients presenting a normal atrio-ventricular conduction. In this stimulation mode, the pacemaker calculates an atrial ventricular delay (“AVD”) of stimulation and detection by analyzing the spontaneous conduction of the patient. This principle of operation authorizes, in the majority of times, the maintenance of a spontaneous ventricular activity. However, this behavior principle is limited by a maximum duration of the conduction delay, about 300 ms after an atrial detection, and 350 ms after an atrial stimulation.
This operating mode is adapted to those patients presenting a sinusal activity but, in the event of sinusal dysfunction requiring an atrial stimulation, it was noted that, on a significant number of cardiac cycles either (i) a ventricular stimulation was started, i.e., no ventricular activity was, or could, be detected, or (ii) a fusion occurred, i.e., a ventricular stimulation intervened in a concomitant way to a spontaneous ventricular depolarization, detected in the same temporal window as the ventricular stimulation. These phenomena are probably due to the fact that atrial stimulation increases the atrio-ventricular delay for conduction, which then exceeds the value of the programmed AVD, typically 350 ms after stimulation.
When one wants to combine the algorithms for the prevention and treatment of atrial arrhythmias with a DDD-AMC stimulation mode, one notes a very significant increase in the number of cycles with a ventricular stimulation. Indeed, the object of these algorithms is to remove the spontaneous atrial activity and to “overdrive” the sinus permanently.
It is also known that, to maximize the benefit of the overdrive algorithms in the prevention of atrial fibrillation (“AF”), it is significant to maintain good atrial hemodynamics. But a premature ventricular stimulation (which may occur because of a too short AVD), preventing the expression of spontaneous ventricular depolarization, modifies the atrio-ventricular sequence and loses the benefit of atrio-ventricular optimization such that, the atrium will not have time to fill (or to eject the blood) completely.